1. Field of the Invention
The present invention relates to a hydraulic servo mechanism for actuating friction engagement devices of an automatic transmission for a vehicle.
2. Description of the Related Art
An automatic transmission for a vehicle which is controlled by a hydraulic controller employs friction engagement devices, such as clutches and brakes, in order to cause a given constituent element to be selectively engaged with and disengaged from another constituent element, thereby attaining a speed change ratio in accordance with vehicle running conditions such as the speed of the vehicle, the degree of throttle opening, the torque of the output shaft, and the set position of a speed selector, for example, a selector lever. This type of friction engagement device is generally engaged and disengaged by means of a hydraulic servo which is constituted by a hydraulic cylinder, a piston slidably fitted in the cylinder and a means for biasing the piston in the direction in which it is returned. When the friction engagement device is to be engaged, the hydraulic servo is supplied with hydraulic oil which is delivered from an oil pump serving as a hydraulic pressure generating source and driven by the output shaft of the engine of the vehicle and which has been adjusted to a predetermined hydraulic pressure by means of an hydraulic pressure regulating mechanism (e.g., a regulator valve) which is provided within the hydraulic pressure controller and controls the delivery pressure of the oil pump in accordance with the above-described vehicle running conditions, this hydraulic oil being fed through oil line switching valves such as a manual valve interlocked with the speed selector and a shift valve which is actuated by an input hydraulic pressure which corresponds to the vehicle running conditions. On the other hand, when the friction engagement device is to be disengaged, the hydraulic oil is discharged through the above-described switching valves. The oil line for supplying and discharging working oil to and from the hydraulic servo is properly provided with an accumulator which is generally constituted by a hydraulic cylinder, a piston slidably fitted in the hydraulic cylinder and a piston return biasing means utilizing a return spring, back pressure or the like, a check valve and a throttle for the purpose of adjusting the rise in hydraulic pressure (pressure-up characteristic) and the decay in hydraulic pressure (pressure-down characteristic) within the hydraulic servo so that the friction engagement device is engaged and disengaged at optimal timings, thereby lowering the magnitude of any impact which may be caused when vehicle speeds are changed.
In general, there are more occasions where it is necessary for engagement of an automatic transmission for a vehicle to be effected more slowly than for disengagement thereof. Furthermore, when the friction engagement device is to be engaged it is first in a sliding state (in a slide engagement) with dynamic friction, but when the friction engagement device is to be disengaged it is necessary, since it has been in an engagement state with static friction, for the hydraulic pressure within the hydraulic servo (referred to as "servo pressure", hereinafter) to be reduced to a sufficiently low pressure to cause the device to start sliding.
The conventional hydraulic servo mechanism, however, suffers from the following disadvantages. Namely, it is conventional practice to set the servo pressure and the hydraulic pressure within the accumulator (referred to as "accumulator pressure", hereinafter) such that they are equal to each other. In consequence, if the capacity of the accumulator, the return force characteristic of the return biasing means and the resistance against the flow of working oil up to the junction of the supply/discharge oil line of the accumulator and that of the hydraulic servo are set such as to be appropriate for the engagement of the friction engagement device, when the friction engagement device is disengaged, the accumulator pressure drops excessively after the optimal timing for the pressure drop, and an unfavorably large shock may be generated when vehicle speeds are changed since the friction engagment device is suddenly disengaged when the piston of the accumulator stops at its end point.